History Of Atomic Theory

Democritus (Greece)

Democritus believed that everything is composed of atoms. He said that atoms are physically but not geometrically indivisible, that they are indestructible, between them there is empty space, they have always been and always will be in motion, there is an infinite number of atoms, infinite kinds of atoms which all differ in shape and size. He also believed that the solidness of the material corresponded to the shape of the atoms involved.
Democritus brought about the theory that atoms exist.

Jan 1, 1704

Isaac Newton (England)

Newton recognised that there are forces between atoms and that they affect the chemical properties of matter.
He helped publicise that atoms affect the chemical properties of matter and helped us understand that atoms are what make everything work.

Jan 1, 1803

John Dalton (England)

Dalton said that elements consisted of tiny particles called atoms, & that the reason an element is pure is because all the atoms of an element are identical, & that they have the same mass. His theory also stated that atoms of each element were different from one another; in particular, they had different masses which helped lead to his conclusion that each atom has its own characteristic weight. He said that compounds were atoms of different elements put together.
This helped classify atoms.

Jan 1, 1894

George Johnstone Stoney (Ireland)

Stoney suggested that a subatomic particle existed which was a particle of electricity held within the atom, named the electron.
This helped us understand that within the atom, there are other particles and that atoms have a charge (as electricity is charged).

Jan 1, 1897

J.J. Thomson (England)

Thomson discovered that atoms did indeed contain particles known as electrons. He discovered this through finding that cathode rays could be deflected by an electric field, thus concluding that these rays were composed of very light negatively charged particles which he called ‘corpuscles’, later to be named ‘electrons’.
This proved Stoney’s hypothesis (and thus other consequences) was correct.

Jan 1, 1900

Max Planck (Germany)

Planck discovered quantum theory, by studying the nature of light and other radiation. He hypothesised at very small scales, that matter was discrete rather than continuous.
This means that we know that atoms behave differently to the human-scale world that we know and perceive.

Jan 1, 1903

Hantaro Nagaoka (Japan)

Nagaoka developed an early, incorrect "planetary model" of the atom which was based around an analogy to the explanation of the stability of Saturn’s rings. It had a massive nucleus, with electrons revolving around the nucleus, bound by electrostatic forces. However, this analogy was wrong although the electrons revolving around the nucleus was correct, and was abandoned in 1908.
This model (although incorrect) helped develop the idea of the nucleus of an atom being orbited by electrons.

Jan 1, 1909

Robert Millikan (USA)

Millikan measured the charge on an electron with his oil-drop apparatus. He also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity.
This means that we know that all the electrons in every atom has the same charge.

Jan 1, 1911

Ernest Rutherford (New Zealand)

Rutherford formulated a model of the atom, which was like this: a very small positively charged nucleus, orbited by electrons. He also speculated on the existence of ‘neutrons’, which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keeping the nuclei from breaking apart.
Rutherford discovered the basic form of an atom regarding protons and electrons.

Jan 1, 1914

Henry G.J. Moseley (England)

Moseley determined the charge of most nuclei using X-rays. He also discovered that an element's atomic number and number of protons were equal, leading to the Periodic Table of Elements being arranged by atomic number instead of atomic mass, which helped organise it much better.
It meant that all atoms of different elements a have different weights.

Jan 1, 1922

Niels Bohr (England)

He suggested that when an atom is excited or heated, electrons can jump to higher levels, & when the atom cools, they drop down to lower levels, where precise quanta of energy are released as specific wavelengths. An electron’s energy levels can be imagined as concentric circles around the nucleus. He predicted the levels (or shells) had a maximum capacity of electrons each, the innermost being 2.
This helps us understand why atoms release light and energy on at certain frequencies.

Jan 1, 1932

James Chadwick (England)

Chadwick discovered a 3rd type of subatomic particle and named it the ‘neutron’. They help stabilize the protons in the atom’s nucleus, preventing the protons from repelling each other. Neutrons are always in the nucleus of atoms, and are about the same size as protons however, they have no electrical charge.
This helps us understand why atoms stay together despite the positive charges of the protons.